Spinal cord injury (SCI) triggers a neuroinflammatory reaction that can aggravate tissue injury (e.g., neuronal death, axonal injury, demyelination) and promote repair (e.g., axon regeneration, remyelination, revascularization). Recently, functionally distinct subsets of macrophages, i.e., M1 (pro-inflammatory) and M2 (anti-inflammatory) cells have been identified at sites of SCI which may underlie the functional dichotomy. M1 macrophages are neurotoxic and dominate the injured spinal cord for several weeks post-injury. In contrast, M2 macrophages promote axon growth, even in the presence of inhibitory molecules (e.g. CSPG and myelin), without concomitant neurotoxicity. Unfortunately, M2 macrophages populate the injury site for only a few days, eventually becoming overwhelmed by M1 macrophages. Thus, the high M1:M2 ratio might explain why repair of the injured CNS is slow and inefficient relative to tissues in the periphery. We predict that the efficiency and magnitude of spinal cord repair will be improved by modulating the phenotype and function of macrophages that respond to the injury. We will test this hypothesis using genetic loss-of-function (knockout) and gain-of-function (lentiviral) techniques to manipulate expression of the triggering receptor expressed on myeloid cells-2 (TREM2) on resident microglia and myeloid precursor cells, i.e., cells that give rise to monocyte-derived macrophages (MDMs), since overexpressing TREM2 in macrophages induces an M2 phenotype. In Aim 1 we will examine how TREM2 overexpression or selective knockout on microglia vs. MDMs effects inflammation, motor recovery and anatomical indices of repair after contusive spinal cord injury. By using lentiviral constructs to overexpress TREM2 on MDMs in a model of dorsal spinal hemisection injury in Aim 2, we will determine if TREM2 manipulation influences macrophage effects on myelin/axon phagocytosis, axon regeneration and axonal retraction or die-back of injured axons. Using a model of focal intraspinal demyelination (lysolecithin) in conjunction with the gain of function protocols, we will determine if manipulating macrophage TREM2 affects OPC differentiation and remyelination within the spinal cord in Aim 3. The current proposal outlines proof-of-principle experiments that will advance our understanding of how a distinct molecular signaling pathway, i.e., TREM2, influences the natural course of CNS macrophage function after SCI. Importantly, if data from these studies indicate that manipulating TREM2 confers anatomical or functional benefits with minimal or no adverse effects on CNS structure or function, then it should be feasible to develop similar protocols for human clinical trials. Indeed, intravenous delivery of bone marrow cells (the primary technique to be used in the proposed studies) has already been tried in SCI patients and without adverse effects. Moreover, enzyme replacement therapies, using lentiviral transduction of autologous peripheral blood mononuclear cells, were shown to be safe and effective in human subjects.